The role of a non‐native tree in riparian vegetation expansion and channel narrowing along a dryland river

Abstract

Along rivers, native and invasive species may establish and persist on active channel bedforms as part of channel narrowing. Using historical aerial photography and dendrochronology, we quantified spatial and temporal patterns of narrowing and vegetation expansion, including native Fremont cottonwood (Populus fremontii) and non‐native Russian olive (Elaeagnus angustifolia), along the largely unregulated Escalante River in south‐western United States. Russian olive establishment was examined with respect to hydrologic and climate variables. Narrowing along the Escalante River was initiated during a mid‐20th century drought. Cottonwood rapidly colonized higher, bar surfaces between the 1950s and 1981. Small numbers of Russian olive established in moist sites during this period as the channel narrowed by nearly 80%. After 1981, there was no obvious cottonwood establishment but low channel bars and banks were rapidly colonized by Russian olive. Hydroclimate predictors were equivocal but exponential growth of this large‐seeded, shade‐tolerant species lagged its introduction by 30 years, apparently because of delayed reproductive maturity, limited seed availability, and widespread availability of favourable establishment sites following initial channel narrowing. Sediment trapping, levee formation, and modification of channel form by dense, channel‐edge bands of Russian olive progressively limited new establishment sites and by 2000, recruitment declined sharply. Our results have implications for management of non‐native tree invasions along arid‐region rivers, including identification of low, moist, active channel bars where the establishment and physical impacts of Russian olive appear to be most pronounced and where focused management efforts are likely to be most effective.

Failure to achieve recommended environmental flows coincides with declining fish populations: Long‐term trends in regulated and unregulated rivers

“…The apparent weakening of previous flow–ecology relationships probably resulted from the combination of these lower magnitude flows during recent periods and more variable fish densities observed during this low flow period. We hypothesise that the variability in fish densities observed during the recent lower flow period may be driven by interactions between lower flows and other non‐flow stressors, such as habitat loss resulting from reductions in total annual discharge and concomitant channel narrowing and homogenisation of in‐stream habitat (Franssen et al, 2014; Scott et al, 2018). Encroachment of riparian vegetation following reductions in flow can contribute to channel narrowing and decreased channel avulsion (Scott et al, 2018; Walker et al, 2020), such as has occurred on the San Juan River (Franssen et al, 2014).…”

“…During this time, native fish densities, including previously common species such as flannelmouth sucker ( Catostomus latipinnis ), bluehead sucker ( Catostomus discobolus ), and speckled dace ( Rhinichthys osculus ) have declined (this study). The reasons for these declines are unknown, but could be linked to flow and non‐flow stressors, including, habitat simplification associated with encroachment of riparian vegetation (Katz & Shafroth, 2003; Scott et al, 2018; Vorster et al, 2018), potential impacts of non‐native fish (Franssen et al, 2014; Hedden et al, 2020; Mueller, 2005), declines in water quality (Hamilton & Buhl, 1997), declining catchment runoff (Milly & Dunne, 2020; Udall & Overpeck, 2017), reductions in large floods and overbank flows (Grams et al, 2020; Lytle & Merritt, 2004), and potential interactions among these variables. The observed declines in native fishes in the San Juan River coinciding with a failure to achieve e‐flow standards necessitate an assessment of the implementation and prioritisation of e‐flow standards in the face of multiple stressors associated with river regulation and climate change.…”

A tale of two rivers: Dam‐induced hydrologic drought on the lower Dolores River and its impact on tamarisk establishment

“…By the early 1990s, after completion of the dam and more irrigation systems coming online, the open bars and channel margins that had previously been created and maintained by spring floods became increasingly vegetated, leading to channel narrowing (Figures 2 & 4). While such narrowing is often linked to invasive species like tamarisk and Russian olive (e.g., Allred & Schmidt, 1999; Birken & Cooper, 2006; Scott et al, 2018), on the Dolores River, it is currently driven by native species, specifically coyote willow and giant reedgrass ( Phragmites australis ) (Dott et al, 2016; TNC, 2018). Because willows favour sites where their roots are in near‐constant contact with the water table (Amlin & Rood, 2002; Douhovnikoff et al, 2005), the increase in low flows during the drier summer months has allowed rapid expansion of the rhizomatous S. exigua .…”

“…Including hydrologic and climatic data for the year of, the year before and the year after establishment accounts for some possible ageing errors of tamarisk stems and also incorporates the potential influence of those factors in years pre-and post-establishment (cf. Scott et al, 2018).…”

Drought in intermittent river and ephemeral stream networks

“…Combined alterations in flow and thermal regimes caused by drought can also favour establishment and dominance of non‐native species of riparian plants (Glenn & Nagler, 2005; Scott et al, 2018), fishes (Bêche et al, 2009; Bernardo et al, 2003; Hopper, Gido, Pennock, Hedden, Frenette, et al, 2020a; Jaeger et al, 2014; Rogosch et al, 2019; Whiterod et al, 2015) and invertebrates (Kouba et al, 2016; Larson et al, 2009) in IRES. Such invasions are facilitated if invasive species are more resilient and resistant to drought than native species.…”